ε is the induced EMF, N is the number of loops you have (in all of our examples, we’ve only had one
loop), and Δt is the time during which your magnetic flux, Φ (^) B , is changing.
Up until now, we’ve just said that a changing magnetic flux creates a current. We haven’t yet told you,
though, in which direction that current flows. To do this, we’ll turn to Lenz’s Law .
Lenz’s Law: States that the direction of the induced current opposes the increase in flux
When a current flows through a loop, that current creates a magnetic field. So what Lenz said is that the
current that is induced will flow in such a way that the magnetic field it creates points opposite to the
direction in which the already existing magnetic flux is changing.
Sound confusing?^4 It’ll help if we draw some good illustrations. So here is Lenz’s Law in pictures.
We’ll start with a loop of wire that is next to a region containing a magnetic field (Figure 20.11a ).
Initially, the magnetic flux through the loop is zero.
Figure 20.11a Loop of wire next to a region containing a magnetic field pointing out of the page.
Now, we will move the wire into the magnetic field. When we move the loop toward the right, the
magnetic flux will increase as more and more field lines begin to pass through the loop. The magnetic flux
is increasing out of the page—at first, there was no flux out of the page, but now there is some flux out of
the page. Lenz’s Law says that the induced current will create a magnetic field that opposes this increase
in flux. So the induced current will create a magnetic field into the page. By the right-hand rule, the
current will flow clockwise. This situation is shown in Figure 20.11b .
Figure 20.11b Current induced in loop of wire as it moves into a magnetic field directed out of the
page.
After a while, the loop will be entirely in the region containing the magnetic field. Once it enters this